A stepped automatic transmission mounted on a vehicle, such as a car, typically includes: a plurality of planetary gear sets (planetary gear mechanisms); and a plurality of oil pressure friction engaging elements such as clutches and brakes. By selectively engaging the friction engaging elements by control of an oil pressure circuit, power transmission paths passing through the planetary gear sets are switched, and as a result, a plurality of forward gear stages and typically one reverse gear stage can be realized.
The oil pressure circuit of this type of automatic transmission includes: a plurality of switching valves configured to switch oil pressure supply passages; and a plurality of solenoid valves (electromagnetic oil pressure control valves) used to adjust oil pressure or open or close an oil passage. When some sort of abnormality (such as valve stick, coil disconnection, or short circuit) occurs at any of these switching valves and solenoid valves, the supply of the oil pressure to a specific friction engaging element through the abnormal switching valve or solenoid valve may not be performed or may not be stopped. When such abnormality occurs, a gear stage different from a gear stage corresponding to a traveling state is mistakenly realized, so that a traveling performance of the vehicle deteriorates.
As a countermeasure against the abnormality of the oil pressure circuit, PTL 1 discloses control of detecting, while gear change is not performed, an occurrence of interlock based on deceleration of the vehicle and a difference between a gear ratio of a command gear stage determined in accordance with the traveling state and an actual gear ratio, the interlock being caused by engagement of a friction engaging element which should not be engaged. When the occurrence of the interlock is detected, so-called limp home control is executed such that: gear change to a predetermined gear stage is performed for realizing traveling to the extent possible; or switching to a neutral state is performed